The present invention relates to improvements in corrugated board manufacture. Typically, such corrugated board is manufactured from two liners, called the single-face liner and double-face liner (or double-backer), and a fluted medium. The fluted medium is first glued to the single-face liner to form a single-face web. This web is then glued to the double-face liner to form the corrugated board. Since the corrugated board cannot be tightly rolled (as can paper when it is manufactured), the board is much more susceptible to warping than other paper products. In particular, the warping is related to the differential average moisture content and the differential transverse moisture content in the two liners. It is therefore desirable to maintain a uniform differential average and transverse moisture content for both liners so that the overall product will have minimal or no warpage.
Although some of the prior art references as disclosed below have been directed to the manufacture of corrugated paper products, they have not incorporated the automatic warp prevention techniques utilized in the present invention. Table 1 is a list of prior art references with a breakdown of these references in five groups. Copies of these references will be made part of the file history of this patent.
TABLE 1 ______________________________________ U.S. Pat. No. Inventor Date of Issue ______________________________________ Group I Corrugated Board and Other Machines With Warp Control Systems 3,004,880 Lord 1961 3,936,665 Donoghue 1976 3,981,758 Thayer et al 1976 Group II Control Systems Used By The Paper Industry 3,596,071 Doering 1971 3,622,448 Adams et al 1971 3,678,594 Goerz, Jr. et al 1972 3,196,072 Wirtz 1965 3,711,688 Stout et al 1973 3,779,843 Knapp 1973 3,994,602 Howarth 1976 4,174,237 Hemming Jr. et al 1979 4,184,204 Flohr 1980 Group III Zig-Zag Zone Scan and Averaging Techniques 3,496,344 Chope 1970 3,508,035 Worthley 1970 3,510,374 Walker 1970 3,562,500 Grant 1971 3,626,165 McCall 1971 3,691,940 Hays et al 1972 Group IV Infrared Sensors 3,150,264 Ehlert 1964 3,662,170 Keyes IV 1972 Group V Wrap Arm Control 4,038,122 DeLigt 1977 4,056,417 League IV 1977 4,071,392 Chaudhuri 1978 ______________________________________
In Group I, U.S. Pat. No. 3,004,880, Lord, describes a conventional corrugator machine in existence during the late 1950's and further describes a control method for preventing transverse warping or curl as shown in its FIGS. 2 and 3. This reference uses radiation pyrometers 192, 194, 196 and 198 to sense the temperature conditions of the upper liner 14, a bottom liner 22, a flute or corrugated medium 18 and the single-face board 72 comprising a combination of the single-face liner and medium. The pyrometers transmit electrical signals proportional to their sensed temperatures (and thus moisture) to controllers 172, 174, 180 and 182 which in turn transmit output signals by way of conductors 200, 202, 204 and 206 to their associated electric motors and gear reduction units 208, 210, 212, and 214 which in turn control the angular position of wrap arms 26, 80, 98 and 44. These wrap arms thus control the amount of drying for the webs moving across the associated heater roller 34, 88, 106 and 52. The pyrometer readings are compared to a set point pressure signal delivered by respective ratio relays 148, 149, 150 and 151, to index mechanisms 170, 175, 168 and 178, and the signals are then sent to each of the controllers.
The disclosed system in Lord requires in one embodiment for an observer to sense curling and then to make adjustments to regulators 127, 128, 129 and 130 so as to change the pressures which in turn are used in conjunction with the pyrometer readings to adjust the wrap angle of the wrap arms. In addition, sensors such as those shown in FIGS. 6 and 7 of Lord can be used to automatically sense either convex or concave transverse warp such as shown in FIGS. 2 and 3. However, the control action is to adjust the wrap arm. No zonal measurements are made, and no zonal corrective action is taken.
Lord also discloses the use of a tachometer generator to sense the speed at which the tension roll 54 is rotating and to transmit an electrical signal proportional to the speed to controller 234. This in turn can adjust the amount of steam spray emitted at header 56 onto medium 18. However, Lord does not disclose or suggest sampling the moisture content transversely along the longitudinally moving medium nor taking weighted zonal time averages to determine appropriate adjustments to corresponding spray heads associated with the moving medium.
Thus, zonal measurements are not made on the single-face liner and double-face liner as in the present invention nor are zonal adjustments made to the moisture content based upon the overall difference of the moisture measurements for corresponding zones. In this latter aspect of the present invention, differential zonal offsets and average offsets are used to determine the amount of corrective action. None of these aspects are disclosed or suggested by Lord.
In addition, the present invention provides overall control of the absolute moisture content of the single-face liner and double-face liner through adjustments in the wrap arms for the respective liners. The present invention also provides for moisture measuring offsets depending upon the weight of the liner being used to form the corrugated board and thereby is able to make adjustments in these moisture measurements when such is necessitated by the moisture sensor's characteristics. None of these aspects of the present invention are either disclosed or suggested by Lord.
U.S. Pat. No. 3,936,665, Donoghue, discloses a sheet material measuring, monitoring and controlling apparatus which utilizes a series of transverse sensors to sense such parameters as density, moisture content, resistivity or other physical or chemical characteristic of the moving sheet. It also discloses use of averaging circuitry 31, 32, and 33 whose average inputs are transferred to a computer curve fitting and profile derivation circuit which uses what is known in mathematics as regression equations to determine the appropriate control to take by a sheet material characteristic controller means 10 (see FIG. 3). In particular, Donoghue discloses taking a series of measurements across a moving sheet material and based upon the sensed values, determining the coefficients of a corresponding regression equation. Once the coefficients are calculated, the value of the parameter for points between the sensed locations can then be computed. The parameter is averaged over the entire transverse length and the computed average compared to desired values so that corrective action can be taken.
This reference does not disclose or suggest transversely scanning the moving sheet but actually suggests that this technique is not as desirable for the applications disclosed. Donoghue further does not disclose or suggest transversely sensing two moving liners, making average zonal measurements, comparing corresponding zonal measurements and taking corrective action in response thereto. Donoghue also does not suggest the use of offsets and variable gain factors for taking corrective action.
Donoghue further does not suggest taking multiple consecutive readings of parameters and asymptotically computing the magnitude of the parameter and thus the amount of corrective action to take.
U.S. Pat. No. 3,981,758, Thayer et al, discloses an apparatus for producing double-face corrugated paperboard webs and for controlling warping. In the background section of this reference, the various factors contributing to corrugated paperboard warp are discussed. The disclosed system has a console with speed indicators and the like with a system programmed to respond to symptoms of poor quality such as warp which are manually observed and indicated to the system. The system then makes the needed corrections and adjustments. These adjustments include preheater web wrap at one of three locations to control the amount of heat applied to the lamina and adjustment of water sprays to control the amount of moisture applied to the lamina. In instances of extreme warp, an adjustment of the thickness of the adhesive applied to the corrugated medium flute tips is made to control both moisture content and overall quality of the final blanks. The present invention's technique for zonally sensing moisture and taking asymptotic time averages thereof to provide appropriate control to the water spray heads is not shown or suggested by Thayer et al. Other aspects of the present invention, including differential offsets, moisture measuring offsets, and variable zonal gain factors (sometimes called damping) are also neither disclosed or suggested by Thayer et al.
The references in Group II are directed to control systems used in the paper industry. They illustrate the various techniques used for sensing and taking corrective actions to a continuous sheet material fabrication apparatus. None of these references disclose or suggest the control method and system of the present invention.
The references in Group III disclose various zig-zag zone scanner averaging devices used in conjunction with the fabrication of various types of sheet material. U.S. Pat. No. 3,562,500, Grant, discloses that a plurality of readings may be taken across a moving sheet and that averages of sampled values can be taken. Grant does not disclose or suggest taking parameter readings over corresponding zonal locations of two liners or to use such information, in combination with offsets, variable gain factors and sensor compensation factors in order to take corrective action. Grant also does not disclose asymptotic measurement of zonal parameters and corresponding corrective action so as to obtain a desired correction with minimal overshoot.
The other patents in this group, although disclosing scanning and averaging techniques, similarly do not disclose the methods and apparatus of the present invention as referred to above.
The Group IV patents, namely U.S. Pat. Nos. 3,150,264. Ehlert, and 3,662,170. Keyes IV, both disclose the use of infrared sensors for measuring the moisture content of paper generated by paper making machinery. Although such sensors are used in the preferred embodiment of the present invention, the other aspects of the present invention as detailed above are neither disclosed nor suggest by these references.
Finally, the references in Group V disclose various apparatus for wrap arm control. In particular, League IV discloses an open-loop digitally operable method and apparatus for controlling the application of heat to webs in preconditioning sections of corrugating machinery as a function of various production factors. The disclosure is directed to producing finished blanks with minimum warpage. The control here utilized is the application of heat to the webs in preconditioning sections of the corrugating machine. Neither zonal parameter sensing of two liners nor the other aspects of the present invention are disclosed or suggested by League IV or any other references in this group. The present automatic warp prevention apparatus and method makes a number of improvements on the prior art so as to result in corrugated board manufacture with increased throughput, higher quality board, and less machine down time.